1. Field of the Invention
The present invention relates to a temperature sensor having a sensing element containing a thermistor and a method of manufacturing the temperature sensor.
2. Description of the Related Art
Conventionally, a temperature sensor has a metal tube including a thermistor and leads. In the prior art, the metal tube is filled with insulating powder. When the metal tube is being filled with insulating powder, the thermistor may be displaced. If such a displacement occurs, the temperature detecting characteristic of the temperature sensor changes so that temperature detection with high accuracy cannot be performed. In particular, if the thermistor sensor touches the metal tube due to the displacement of the thermistor, the resistance of the thermistor greatly changes so that accurate detection of temperature cannot be performed.
An object of the present invention is to provide a temperature sensor capable of correctly detecting temperature and a method of manufacturing such a temperature sensor.
In order to accomplish the objects, according to a first aspect of the present invention, there is provided a method of manufacturing a temperature sensor having a sensing element consisting of a thermistor and leads connected to the thermistor and a sheath pin incorporating core wires. The method includes the steps of connecting the leads to the core wires, arranging an insulator around the sensing element, filling the insulator with an inorganic adhesive to fix the sensing element in the insulator, arranging a metal cover around the insulator, and joining an end of the metal cover to an end of the sheath pin.
The insulator may have an opening through which the inorganic adhesive is poured in. For example, the insulator may have an open end through which the inorganic adhesive is injected into the insulator.
Alternatively the insulator may have a cut at a proper position thereon so that the inorganic adhesive may be injected into the insulator through the cut.
The insulator and the metal cover both may have a shape of a cylinder.
The thermistor may be made of exothermic resistance material whose resistance varies in response to temperature. For example, it may be made of a Crxe2x80x94Mnxe2x80x94Al oxide or a silicon carbide.
According to the manufacturing method of the first aspect of the present invention, the insulator is arranged around the sensing element, and the insulator is filled with the inorganic adhesive to fix the sensing element in the insulator.
The inorganic adhesive has a fluidity of paste state or the like so that it fills the inside of the insulator without displacing the sensing element. When the inorganic adhesive dries and solidifies, it strongly fixes the position of the sensing element in the insulator.
The insulator around the sensing element secures insulation between the metal cover and the thermistor, thereby preventing the resistance of the thermistor from fluctuating due to external factors. As a result, the temperature sensor correctly measures temperature.
In this way, the temperature sensor manufactured according to the method of the present invention correctly detects temperature.
Since the inside of the insulator is filled with the inorganic adhesive, the sensing element never moves in the insulator. Accordingly, the sensing element is not easily damaged and shows high durability.
The insulator may be made of electric insulation material. For example, it may preferably be made of an oxide-based ceramic such as alumina, mullite, and zirconia, a silicon nitride, or a silicon carbide.
This provides the temperature sensor with high thermal resistance.
According to the second aspect of the present invention, the step of arranging the metal cover around the insulator is preferably carried out by inserting the insulator within the interior of the metal cover after the step of filling the insulator with the inorganic adhesive.
In recent years, a smaller-sized temperature sensor has been desired for use in a detecting system for detecting, for example, a temperature of a catalyser required by regulations concerning exhaust gases. In such a small-sized temperature sensor, the diameter of each of the leads is very small. Therefore, in the small-sized temperature sensor, the leads can be easily cut due to external vibration or impacts which affect various portions to the temperature sensor.
According to the second aspect of the present invention, the inorganic adhesive is filled within the interior of the insulator, and then the insulator is inserted into the interior of the metal cover. As a result, the leads can be fixed with respect to the insulator so that the mechanical vibration of the leads can be prevented. Therefore, even when the leads each having a very small diameter are employed, they are not easily cut.
According to the third aspect of the present invention, the insulator has an open end, and the step of filling the insulator with the inorganic adhesive is carried out by pouring the inorganic adhesive through the open end of the insulator.
As mentioned above, a small sized temperature sensor has been desired in recent years. Such a temperature sensor has the insulator with a small diameter, however, since the end of the insulator is largely opened to have an open end, the inorganic adhesive can be filled through the open end. Therefore, the filling work can be easily performed.
It may also be possible to have an insulator having a shape of cylinder and having both ends opened.
According to the fourth aspect of the present invention, the insulator may preferably have a cut.
When the inorganic adhesive is poured into the insulator through an open end thereof, the cut acts as an air hole to make the adhesive pouring work easier.
The cut may be used to inject the inorganic adhesive into the insulator. In this case, the open end of the insulator acts as an air hole to make the adhesive injection work easier.
A plurality of cuts may also be formed on the insulator. In this case, one of them is used to inject the inorganic adhesive into the insulator, and the others serve as air holes to make the adhesive injection work easier.
According to the fifth aspect of the present invention, the inorganic adhesive may preferably be made of ceramic-based powder and an inorganic binder.
This secures high insulation and vibration resistance for the temperature sensor.
The ceramic-based powder may be made of an oxide-based ceramic such as alumina, mullite, and zirconia, a silicon nitride, or a silicon carbide.
The binder may be water glass or an inorganic binder whose main component is silica.
As the above-mentioned inorganic adhesive, it is preferable to use the one having a coefficient of thermal expansion close to that of the thermistor element or the insulator. By employing such an inorganic adhesive, the generation of cracks in the inorganic adhesive due to the difference in thermal expansions in a high temperature environment can be prevented.
According to the sixth aspect of the present invention, there is provided a temperature sensor consisting of a sensing element composed of a thermistor and leads extending from the thermistor and a sheath pin containing core wires. The leads are connected to the core wires. The sensing element is surrounded with an insulator that is filled with an inorganic adhesive. The insulator is surrounded with a metal cover. An end of the metal cover is joined to an end of the sheath pin.
The insulator around the sensing element secures insulation between the thermistor and the metal cover, thereby preventing the resistance of the sensing element from fluctuating due to external factors.
Consequently, the temperature sensor correctly detects temperature.
In this way, the temperature sensor of the present invention is capable of accurately measuring temperature.
Since the leads are fixed to the insulator by the inorganic adhesive, the leads cannot easily be cut due to external vibration or impact.
This effect can be obtained in particular when the diameter of each of the leads is 0.5 mm or below, or when the outer diameter or the metal cover is 4 mm or below.
According to the seventh aspect of the present invention, the diameter of each of the leads is preferably 0.5 mm or below and a lowering of the performance of response due to an increase in calorific capacity can be prevented.
From the point of view of keeping the strength against vibration, the lower limit of the diameter of the lead is preferably 0.1 mm.
According to the eighth aspect of the present invention, the outer diameter of the metal cover is 4 mm or below. Whereby an effect of a high speed response can be attained.
From the point of view of protecting against vibration, the lower limit of the diameter of the metal cover is preferably 1 mm.